Liquid crystal driving and switching apparatus utilizing multivibrators and bidirectional switches

ABSTRACT

A liquid crystal or other display device employing a drive and control circuit for activating selected segments of the display with a bidirectional current utilizing integrated circuit technology with a low voltage DC primary power supply to produce and control the bidirectional current. Accordingly, said DC power supply is utilized to operate a multivibrator circuit which at first and second outputs thereof provides rectangular waveforms varying alternately between a first and a second DC level to cause bidirectional current flow through the crystal when a return path is implemented through a bidirectional current conducting switch.

7 I M M, 3 gm I! United States 1 3,744,049 Luce July 3, 1973 LIQUIDCRYSTAL DRIVING AND 3,505,804 4/1970 Hofstein 340/336 SWITCHINGAPPARATUS UTILIZING MULTIVIBRATORS AND BIDIRECTIONAL SWITCHES PrimaryExaminerJohn W. Caldwell Assistant Examiner-Marshall M. CurtisAttorney-Harvey W. Mortimer. Robert R. Keegan [75] Inventor: Nunzio A.Luce, Trenton, NJ. et a] [73] Assrgnee: Optel Corporation, Princeton,NJ. 57] ABSTRACT [22] Filed: 1971 A liquid crystal or other displaydevice employing a [21] Appl. No.: 199,181 drive and control circuit foractivating selected segments of the display with a bidirectional currentutiliz- [52] U S Cl 340/336 340/324 R 350l160 LC ing integrated circuittechnology with a low voltage DC [51] In Cl G09 9/30 primary powersupply to produce and control the bidi- [58] Fie'ld 160 LC rectionalcurrent. Accordingly, said DC power supply /324 R M is utilized tooperate a multivibrator circuit which at first and second outputsthereof provides rectangular waveforms varying alternately between afirst and a [56] References Cited second DC level to cause bidirectionalcurrent flow UNITED STATES PATENTS through the crystal when a returnpath is implemented 3,575,492 4/1971 Nester et al. 350/160 LC through abidirectional current conducting switch, 3,622,224 ll/l97l Wysocki etal. 350/160 LC 3,653,745 4/1972 Mao 350/160 R 15 Claims, 4 DrawingFigures 1? l. 3T"I-\T'G INT TJT? 7 l v 33 l DECODING 8 L COMPUTATION 53I 58 GI 62 59 e LOGIC l l 63 /80 I 67 l d. F M i 1 ozaae FIG. 1

FIG. 2

lllilllllll PUTS 22 COMPUTATION LOGIC FIG. 4

3| +v 6 IN EIIJ: I oecoome a I w U I IIIII G. Q WEN 7 m m am m y w I mwwT n P .1. 6 mm m. m N Dc (I 9 E m n BL F I/ w w w e w I .T 5 K %m M M If1?. M m. 5 Q I u C" 9 w 7 I Ir mm m I m. a a2 g n ll 9 O H l W l wild.HQ; a m J v w 6 Y. .5 6 3 6 m s. 05 A m m y r|.1I a K no. I L05.

I I I I l I I l l I l I I L LIQUID CRYSTAL DRIVING AND SWITCHINGAPPARATUS UTILIZING MULTIVIBRATORS AND BIDIRECTIONAL SWITCHES Thisinvention relates to liquid crystal or other displays. A typical liquidcrystal display consists of a thin layer of normally transparent liquidcrystal material between two parallel electrodes at least one of whichis segmented. In any area of the liquid crystal material in which a lowvoltage electrical field is applied, the material becomes turbulent,reflecting and scattering ambient light. At least one of the electrodesand the display window are transparent thus forming a very effectivedisplay device which maintains excellent brightness and contrast underhigh ambient light conditions although requiring extremely low power. Ifdesired, conventional external illumination can be provided for thedisplay.

The present invention provides such a display having a driving andcontrol circuit which permits the selection of high contrast liquidcrystal materials with low power requirements and extremely long lifewhile still maintaining the capability of extreme miniaturization andcompatibility with a low voltage direct current primary power supply.

Liquid crystal displays are characteristically controlled by electroniccomputer logic circuits. For example numerical displays of the liquidcrystal type commonly employ a decoder control circuit to convert abinary coded control signal to an appropriate combination of signals toactivate the segments of the display to form a desired arabic numeral.It is most convenient to utilize integrated circuit computer technologyfor the direct control of liquid crystal displays since the currentrequirements of the displays are low enough to present no problem fromthe point of view of current demand. Such electronic computer logiccircuitry almost invariably employs unidirectional current.

However, when conventional integrated circuit computer logic (or in factother conventional computer logic) is used to directly control liquidcrystal displays, one encounters a serious and heretofore unsolvedproblem. The most desirable liquid crystal displays from the point ofview of low power requirements, good contrast and the like undergo apolarization effect when exposed for a long time to unidirectionalcurrent. Hence, a display utilizing unidirectional current and the mostdesirable forms of liquid crystal materials deteriorates in a shortperiod of time, for example on the order of a thousand hours or less. Onthe other hand, it is known that the use of alternating current forliquid crystal displays can readily provide displays with reliablelifetimes of many tens of thousands of hours.

For some applications such as sports scoreboards or the like whereminiaturization is not an objective, the displays may be renderedlong-lived by operating them from a 60-cycle AC power supply andutilizing conventional AC switching techniques in conjunction withappropriate DC computer logic elements to control the switches. This isclearly not an appropriate solution for many applications, such asWristwatches and portable calculators, in which miniaturization is animportant, if not the most important, requirement. Consequently, thoseworking in this technology have engaged in a difficult search for liquidcrystal material for displays which would not deteriorate rapidly whencontrolled by conventional unidirectional integrated circuit computerlogic. That search has not been remarkably successful, and it is oneobject of this invention to make it unnecessary to developunidirectional current driven liquid crystal materials which retain thecapability for miniaturization, long life and high contrast which theliquid crystal display should provide.

The present invention provides a control system for liquid crystaldisplays totally compatible with integrated circuit electronics whereinthe signals controlling and powering the display segments arebidirectional, symmetric current signals.

It is an object of the present invention to provide a liquid crystaldisplay with a low voltage DC primary power supply wherein the displaycells are not exposed to significant unidirectional current flow.

It is another object of the present invention to provide such a liquidcrystal display utilizing integrated circuit technology wherein thedriving and control circuitry for the display may be integrated withpart or all of the logic circuitry called for by the displayapplication.

Other objects and advantages of the apparatus will be apparent fromconsideration of the following description in conjunction with theappended drawings, in which:

FIG. 1 is a plan view of a liquid crystal digital display in accordancewith the invention.

FIG. 2 is an elevational view of the apparatus of FIG. 1.

FIG. 3 is a schematic circuit diagram of one form of integrated circuitin accordance with the invention, and

FIG. 4 is a simplified schematic circuit diagram of an alternative formof integrated circuit for controlling and driving the display inaccordance with the invention.

A liquid crystal reflective numeric display 11 is shown in FIG. 1. Thedisplay, when provided with a low voltage direct current primary powerand conventional digital computer control signals, will display thearabic numerals corresponding to the digital control signals. Asillustrated, display 11 includes three decimal digits and threecontrollable decimal points for the display of numbers from 0.001through 999 with three significant figures.

Obviously, the information capability of the display can be expanded ordiminished as required, and alphabetic or other information can bedisplayed in place of or in addition to numeric information as required.The particular unit illustrated in FIG. 1 may readily be ganged todisplay numerals with greater numbers of significant digits in multiplesof three.

The display unit 11 comprises a display module 13 and a logic module 15.As is known, the display module 13 consists of a cell containing a thinlayer of normally transparent liquid crystal material between oneunitary electrode 27 and a plurality of segmentary electrodes 17.Electrode 27 is the front electrode and is transparent. When an electricfield is applied between the electrode 27 and any one of the segmentaryelectrodes 17, the liquid crystal material therebetween becomesturbulent, reflecting and scattering ambient light. The back of the cellis preferably dark (light absorbtive) for this reflective form ofdisplay to enhance the contrast.

The segmentary electrodes 17 are arranged to form numeral units 19, 21and 23. Circular electrodes 25 provide the controllable decimal pointindicators. Suitable constructions for the display module 13 are wellknown, and since the particular form of the display module is notcritical to the present invention, this construction will not bedescribed in detail. Display module constructions are shown, forexample, in US. Pats. Nos. 3,499,112, 3,499,702 and 3,540,796, andLiquid Crystal Matrix Displays in Proceedings of the IEEE, November1971, Volume 59, Number 11, page 1566.

The logic circuitry for the display is preferably contained in theintegrated circuit module although physically separate units couldobviously be employed. The module 15 is provided with a number ofelectrical leads including control leads 29 and power leads 31.

The integrated logic circuitry of module 15 is illustrated schematicallyin FIG. 3. For clarity of illustration, the display 11 is presented in aform which accepts standard binary coded decimal input signals to getherwith decimal point locator signals and performs all necessarycomputation and decoding to provide a display corresponding to theseinput signals. It should be understood, however, that in manyapplications it will be found convenient to include more extensivecomputation logic in the display 11. For example, the computation logicmay include one or more pulse counters so that the display wouldcorrespond to the number of pulses provided to an appropriate input orto some function thereof.

In an advantageous form of the invention, the decoding and computationlogic and the bidirectional current generator circuit are all formed ona single integrated circuit chip. In this manner, the circuitry can bereduced in size to a fraction of a square inch, at the same timereducing the circuit power requirements to an extremely low level.

Accordingly, in FIG. 3, the integrated circuit chip is represented by adashed enclosure 51. A portion of the chip will be occupied by decodingand computation logic 53 of conventional form. The inputs to the chipare supplied on leads 29 and 31 as previously described, with leads 31being the primary power supply leads. As previously explained, theprimary power supply may be a DC low voltage (less than volts) sourcesuch as a dry cell battery. Other sources or voltages may be employedwhere convenient.

One portion of the integrated circuit chip, shown as the left portion inFIG. 3, is the bidirectional display driver circuit 55. Driver circuit55 may be a freerunning astable multivibrator circuit of conventionalform as illustrated in FIG. 3.

Transistors 56 and 57 are the active elements of circuit 55. Theoperation of the circuit is such that one of the transistors 56 and S7is of while the other of the transistors 56 and 57 is on." Thiscondition is switched at a generally constant rate determined by thecircuit parameters. The circuit is relatively uncomplicated consistingof the transistors, collector resistors 58 and 59, variable baseresistors 61 and 62 with fixed base resistors 63 and 65, and couplingcapacitors 67 and 69.

The operation of free-running astable multivibrator circuits is wellknown, and adaptation of such circuits for the invention is quitesimple. As previously explained, it is important for the driver circuitto operate substantially symmetrically, for example within 10 percent orpreferably within 1 percent. The frequency of the multivibratoroperation is not critical and may be, for example, from 5 to 1,000hertz. Resistors 61 and 62 in the circuit are variable resistors tofacilitate adjustment of the circuit although this would not benecessary if all parameters of the circuit could be preciselycontrolled. It will be understood that any adjustment to the drivercircuit 55 is merely a production expedient and no adjustment isrequired during operation.

The driver circuit 55 as thus far described will operate at a frequencywhich is not well regulated and subject to changes of temperature,voltage or the like. If desired, and particularly if it is readilyavailable, a clock oscillator or synchronizing source 81 may be utilizedto achieve better regulation of the frequency of driver circuit 55. Suchan optional clock or synchronizing source 81 is shown in dashed lines inFIG. 3 connected through an input resistor 83 to the base of transistor56. It should be emphasized that the clock 81 is not essential as therewill normally be no necessity to accurately regulate the driver circuitfrequency.

It will be seen from FIG. 3 that when transistor 57 is on, output lead72 from circuit 55 will be near ground potential, and when transistor 57is off, output lead 72 will be at positive potential. The oppositesituation will prevail for output lead 71.

Accordingly, leads 71 and 72 constitute a source of symmetricbidirectional current available to drive the liquid crystal reflectivedisplay 13. The waveform supplied is essentially square. Lead 72 isconnected to electrode 27 of display 13, and lead 71 is connected to abus which is selectively connectable to segmented electrodes 17 of thedisplay 13. It will be understood that the particular driver circuit 55consisting of a freerunning astable multivibrator circuit is utilizedfor illustration only, and that any circuit capable of being realized onan integrated circuit chip and providing the desired bidirectionalsymmetric current source may be utilized in accordance with theinvention. Some examples of other circuits are P-channel or N-channel orcomplementary P and N channel devices used to construct multivibrator orother bidirectional current sources.

The flow of current from the respective segmented electrode 17 to thecommon electrode 27 is controlled by symmetrical semiconductor controlswitches schematically illustrated at 73, 75, 77 and 79. These switchesare symmetrical in the sense that they will control either half of thebidirectional current driver waveform in the same manner when suppliedwith a given input signal from the decoding and computation logic 53.Various symmetrical integrated circuit switch devices are known, forexample a P-channel and an N- channel field effect transistor withoppositely polarized inputs obtained by the use of an inverter.Alternatively, the switches 73, 75, 77 and 79 may each be a compositecircuit built up from non-symmetrical switching devices in astraightforward manner, although this would require a capacitor in line72.

Reviewing the operation of the device, particularly as schematicallyillustrated in FIG. 3, it will be seen that the inputs to the decodingand computation logic 53 cause certain outputs on leads 80 which, inturn, are the control signals for symmetrical semiconductor switchingelements 73, 75, 77 and 79. Each of such switch elements permits currentflow only when supplied with an on" signal, thus causing the liquidcrystal matrial contiguous to the associated segmented electrode 17 tobe opaque in response to the on" signal.

The current flow between the segmented electrode 17 and the commonelectrode 27 is bidirectional and symmetrical notwithstanding the DCprimary power supplied to leads 31. The operation of the drive circuit55 is such as to sequentially reverse the operative connection ofconductors 70 and 72 (hence the segmented and common electrode) relativeto the positive and negative (ground) terminals 31. This has the very.important consequence of achieving very long life and high reliabilityfor the display utilizing known liquid crystal materials characterizedby intolerance to prolonged unidirectional current flow.

Note that liquid crystal displays may comprise a matrix formed by twosets of segmented electrodes in which case bidirectionalcurrent switcheswould control current to the electrodes of both sets.

An alternative embodiment of the invention is illustrated in FIG. 4wherein the driver circuit is a bistable multivibrator which requires aclock pulse input.

As in FIG. 3, the circuitry is preferably instrumented on a singleintegrated circuit chip indicated schematically at 151. The decoding andcomputation logic is schematically illustrated by block 153 and issimilar to that shown in FIG. 3 except that a clock pulse output 191 isavailable to control the driver circuit 155. Such pulse may be generatedin block 153 or supplied through a lead 129. The driver circuit 155includes a pair of transistors 156 and 157 in a symmetric circuitarrangement further including collector resistors 158 and 159, baseresistors 161 and 162, and a pair of RC coupling circuits 166, 167 and168, 169. Clock pulse lead 191 is connected through capacitor 143 andresistor 141 to the collector transistor 156, and similarly fromcapacitor 144 and resistor 142 to the collector of transistor 157. Adiode 145 is connected from the junction of resistor 142 and capacitor144 to the base of transistor 157, and a diode 146 is connected from thejunction of resistor 141 and capacitor 143 to the base of transistor156.

The bistable multivibrator circuit 155 operates in a known mannercharacterized by one of the pair of transistors 156, 157 being off whilethe other transistor is on. Switching from one condition to the othercondition does not occur spontaneously, however, but is caused by thereceipt of a clock pulse from clock pulse lead 191. The oscillationfrequency of the multivibrator circuit 155 is accordingly controlled(within limits) by the frequency of the clock pulse 191. As before, thisfrequency is not critical and may be set within the range ofapproximately 5 to 1,000 hertz. Assuming that the clock pulse at lead191 is of normal regularity, the symmetry of the oscillation of thebistable circuit 155 will be quite well within the required accuracy.Such bistable circuits may be P-channel or N-channel or complementary Pand N channel devices used to construct a multivibrator or otherbidirectional current source controlled by an external oscillator. Thebidirectional source may also consist of two inverter circuits connectedin series where the output of the first inverter is connected to line172 and the output of the second inverter is connected to line 171 andthe input to the first inverter is driven by an available symmetricalwaveform.

The output from output leads 171 and 172 in FIG. 4 is substantially thesame as from leads 71 and 72 in FIG. 3, and the operation of theremaining portion of the circuit of FIG. 4 is substantially as explainedwith reference to FIG. 3, that is, switches 173, 175 and 177 perform thesame function as switches 73, '75, 77 and 79 to control the operation ofthe liquid crystal display electrodes 117 and 127 as was done to controlthe display with electrodes 17 and 27 in FIG. 3.

It should be noted that for simplification of the illustrations, lessthan all of the inputs 29 and 129, the switches 73, 75, etc., theelectrodes 17 and 117 have been shown. It will be understood that thenumbers of such elements will vary according to the particularapplication. There would, for example, be 24 electrodes 17 and 117 inthe particular embodiment illustrated, each having an associated switchelement 73, 75, etc. The number of inputs 29 would depend on theparticular form of decoding and computation logic but in a typical casemight number 15.

It will be appreciated that the invention is by no means limited to theparticular embodiment of a threedigit numerical display or to theparticular circuitry illustrated, but that the principles of theinvention may be applied generally to liquid crystal displays controlledand driven by integrated circuits.

In addition to the particular embodiments of the invention described andillustrated, it will be apparent that modifications and variationsthereof will be readily devised by those skilled in the art, and thescope of the invention is not limiJed to the particular embodimentsshown or suggested but encompasses the expected variations andmodifications that may be derived from the invention, and it is intendedthat the appended claims be so construed.

What is claimed is;

1. In a display wherein a display cell has discrete portions which arerendered less transparent to generate an image which is a function of aplurality of input signals, the combination comprising a cell containinga liquid crystal material, a first electrode unit in said cell, a secondelectrode unit consisting of a plurality of electrode segments in saidcell, circuit means including a plurality of active circuit elements andincluding a drive current bus, a plurality of bidirectional currentswitches responsive to said input signals each connected in seriesbetween a respective one of said electrode segments and said drivecurrent bus, a current conductor connected to said first electrode unit,two low voltage DC primary power terminals and a plurality of currentswitches in a circuit connected to reversibly establish a currentconnection between respective ones of said power terminals and saiddrive current bus and said current conductor.

2. Apparatus as claimed in claim 1 further including in said circuitmeans a plurality of active circuit elements forming a circuit having asinputs the control inputs for said display device and with outputsconnected respectively to control said bidirectional current switches,said circuit thereby defining the functional relation between saiddisplay inputs and the image.

3. Apparatus as claimed in claim 2 further including in said circuitmeans at least one active circuit element in a circuit forming a clocksignal generator of substantially constant frequency connected tocontrol the reversing of connections from said power terminals to besynchronized with said clock signal.

4. Apparatus as claimed in claim 1 wherein said air cuit connected toreversibly establish connection between said power terminals and saiddrive current bus and said current conductor causes the bidirectionalityof current to be symmetrical within about 10 percent.

5. In a display device wherein a display cell has discrete portionswhich are rendered less transparent to generate an image which is afunction of a plurality of input signals, the combination comprising acell containing a liquid crystal material, a first electrode unit insaid cell, a second electrode unit consisting of a plurality ofelectrode segments in said cell, and at least one integrated circuitdevice having a plurality of active circuit elements integrated thereinand including a drive current bus, a plurality of bidirectional currentswitches responsive to said input signals each connected in seriesbetween a respective one of said electrode segments and said drivecurrent bus, a current conductor connected to said first electrode unit,two low voltage DC primary power terminals and a plurality of currentswitches in a circuit connected to sequentially establish two differentcurrent connections from said power terminals to said drive current busand said current conductor, the first of said connections connecting thefirst power terminal to said drive current bus and the second powerterminal to said current conductor, the second of said connectionsconnecting the first power terminal to said current conductor and thesecond power terminal to said drive current bus.

6. Apparatus as claimed in claim 5 further including in said integratedcircuit device a plurality of active circuit elements forming a circuithaving as inputs the control inputs for said display device and withoutputs connected respectively to control said bidirectional currentswitches, said circuit thereby defining the functional relation betweensaid display inputs and the reflective image.

7. Apparatus as claimed in claim 6 further including in said integratedcircuit device at least one active circuit element in a circuit forminga clock signal generator of substantially constant frequency connectedto control the reversing of connections from said power terminals to besynchronized with said clock signal.

8. Apparatus as claimed in claim 5 wherein said circuit connected tosequentially establish two different current connections from said powerterminals sequences the connections so that their time duration is aboutequal.

9. In a display device wherein a display cell has discrete portionswhich are rendered less transparent to generate an image which is afunction of a plurality of input signals, the combination comprising acell containing a liquid crystal material, a first electrode unit insaid cell, a second electrode unit consisting of a plurality ofelectrode segments in said cell, a drive current bus, a plurality ofcurrent switches responsive to said input signals each connected inseries between a respective one of said electrode segments and saiddrive current bus, a current conductor connected to said first electrodeunit, two DC primary power terminals, and a circuit including aplurality of current switches connected to reversibly establish acurrent connection between respective ones of said power terminals andsaid drive current bus and said current conductor.

10. [n a liquid crystal display of the type including a commonelectrode, a plurality of electrode segments and a layer of a liquidcrystal material interposed between said common electrode and saidelectrode seg ments, said layer being capable of changing its responseto light upon activation by a current, the improvement therewith ofapparatus for activating said liquid crystal with a bidirectionalcurrent, comprising:

a. a multivibrator circuit including first and second active deviceseach having a common electrode, an

input electrode and an output electrode and feedback means coupling theoutput electrode of said first device to the input electrode of saidsecond device and the output electrode of said second device to theinput electrode of said first device, to enable said circuit to beselectively activated to any one of two states wherein the first statesaid first device is biased on and said second device is biased off andin a second state, said first device is biased off and said seconddevice is biased on,

b. first means coupled to at least one input electrode of said first andsecond active devices to selectively alternate said states at a desiredrate, and

c. second means for coupling one of said output electrodes of said firstand second devices to said common electrode of said liquid crystal, andsaid other output electrode to at least one of said electrode segmentsto bidirectionally activate said liquid crystal according to saiddesired rate.

11. The apparatus according to claim 10 wherein said multivibratorcircuit is a bistable multivibrator curcuit.

12. The apparatus according to claim 10 wherein said multivibratorcircuit is an astable multivibrator circuit.

13. In a liquid crystal display of the type including a commonelectrode, a plurality of electrode segments and a layer of a liquidcrystal material interposed between said common electrode and saidelectrode segments, said layer being capable of changing its response tolight upon activation by a current, the improvement therewith ofapparatus for activating said liquid crystal with a bidirectionalcurrent, comprising:

a. a multivibrator circuit having a first and second output terminal ofthe type providing a first given rectangular waveshape at said firstoutput terminal of an amplitude varying between a first DC level and asecond DC level which second level is relatively at reference potential,and at second output terminal providing an analagous waveform of anamplitude at said second DC level when said first waveshape is at saidfirst level and at a said first DC level when said first waveshape is atsaid second DC level, and

b. means coupling said first output terminal to said common electrode ofsaid liquid crystal and said second output terminal to at least one ofsaid electrode segments to cause a bidirectional current to flow throughsaid liquid crystal according to said first and second waveshapes.

14. In a display of the type utilizing a liquid crystal having a commonelectrode, a plurality of segment electrodes and a liquid crystalmaterial located between said common electrode and said segmentelectrodes, the improvement in combination therewith of apparatus forproviding a bidirectional current flow through said liquid crystal toactivate the same, comprising:

a. a multivibrator circuit having a first and second output terminal forproviding a first waveform at said first terminal of an amplitudealternately varying between a first DC level and a second DC level,which second level is relatively at reference potential, and a secondwaveform at said second terminal which alternately varies opposite tosaid first waveform whereby when said first waveform is at said first DClevel, said second waveform is at said second DC level,

b. means coupling said first output terminal of said multivibrator tosaid common electrode of said liquid crystal,

c. means including a bidirectional current switch of the type exhibitinga low impedance in a first state upon activation of said switch by acontrol signal, and a high impedance in a second state, said switchcoupled in series between at least one of said segment electrodes and apoint of reference potential, and

(1. control means coupled to said switch for providing said controlsignal to cause said switch to exhibit said low impedance state toprovide a bidirectional current return for said segment electrode.

15. Apparatus for driving a display of the type including a liquidcrystal having at least two electrodes and a liquid crystal materialbetween said electrodes, said apparatus comprising:

a. first and second active devices, each having an input, output andcommon electrode,

b. means coupling said common electrode to a point of referencepotential,

. first feedback means coupling said output elecd. means coupling saidoutput electrode of said first device to one of said electrodes of saidliquid crystal and said output electrode of said second device to saidother electrode of said liquid crystal to cause a signal to be developedacross said liquid crystal according to the states of said activedevices, and

. means coupled to at least one of said control electrodes of saidactive devices to determine the rate at which said devices are biased onand off.

2. Apparatus as claimed in claim 1 further including in said circuitmeans a plurality of active circuit elements forming a circuit having asinputs the control inputs for said display device and with outputsconnected respectively to control said bidirectional current switches,said circuit thereby defining the functional relation between saiddisplay inputs and the image.
 3. Apparatus as claimed in claim 2 furtherincluding in said circuit means at least one active circuit element in acircuit forming a clock signal generator of substantially constantfrequency connected to control the reversing of connections from saidpower terminals to be synchronized with said clock signal.
 4. Apparatusas claimed in claim 1 wherein said circuit connected to reversiblyestablish connection between said power terminals and said drive currentbus and said current conductor causes the bidirectionality of current tobe symmetrical within about 10 percent.
 5. In a display device wherein adisplay cell has discrete portions which are rendered less transparentto generate an image which is a function of a plurality of inputsignals, the combination comprising a cell containing a liquid crystalmaterial, a first electrode unit in said cell, a second electrode unitconsisting of a plurality of electrode segments in said cell, and atleast one integrated circuit device having a plurality of active circuitelements integrated therein and including a drive current bus, aplurality of bidirectional current switches responsive to said inputsignals each connected in series between a respective one of saidelectrode segments and said drive current bus, a current conductorconnected to said first electrode unit, two low voltage DC primary powerterminals and a plurality of current switches in a circuit connected tosequentially establish two different current connections from said powerterminals to said drive current bus and said current conductor, thefirst of said connections connecting the first power terminal to saiddrive current bus and the second power terminal to said currentconductor, the second of said connections connecting the first powerterminal to said current conductor and the second power terminal to saiddrive current bus.
 6. Apparatus as claimed in claim 5 further includingin said integrated circuit device a plurality of active circuit elementsforming a circuit having as inputs the control inputs for said displaydevice and with outputs connected respectively to control saidbidirectional current switches, said circuit thereby defining thefunctional relation between said display inputs and the reflectiveimage.
 7. Apparatus as claimed in claim 6 further including in saidintegrated circuit device at least one active circuit element in acircuit forming a clock signal genErator of substantially constantfrequency connected to control the reversing of connections from saidpower terminals to be synchronized with said clock signal.
 8. Apparatusas claimed in claim 5 wherein said circuit connected to sequentiallyestablish two different current connections from said power terminalssequences the connections so that their time duration is about equal. 9.In a display device wherein a display cell has discrete portions whichare rendered less transparent to generate an image which is a functionof a plurality of input signals, the combination comprising a cellcontaining a liquid crystal material, a first electrode unit in saidcell, a second electrode unit consisting of a plurality of electrodesegments in said cell, a drive current bus, a plurality of currentswitches responsive to said input signals each connected in seriesbetween a respective one of said electrode segments and said drivecurrent bus, a current conductor connected to said first electrode unit,two DC primary power terminals, and a circuit including a plurality ofcurrent switches connected to reversibly establish a current connectionbetween respective ones of said power terminals and said drive currentbus and said current conductor.
 10. In a liquid crystal display of thetype including a common electrode, a plurality of electrode segments anda layer of a liquid crystal material interposed between said commonelectrode and said electrode segments, said layer being capable ofchanging its response to light upon activation by a current, theimprovement therewith of apparatus for activating said liquid crystalwith a bidirectional current, comprising: a. a multivibrator circuitincluding first and second active devices each having a commonelectrode, an input electrode and an output electrode and feedback meanscoupling the output electrode of said first device to the inputelectrode of said second device and the output electrode of said seconddevice to the input electrode of said first device, to enable saidcircuit to be selectively activated to any one of two states wherein thefirst state said first device is biased on and said second device isbiased off and in a second state, said first device is biased off andsaid second device is biased on, b. first means coupled to at least oneinput electrode of said first and second active devices to selectivelyalternate said states at a desired rate, and c. second means forcoupling one of said output electrodes of said first and second devicesto said common electrode of said liquid crystal, and said other outputelectrode to at least one of said electrode segments to bidirectionallyactivate said liquid crystal according to said desired rate.
 11. Theapparatus according to claim 10 wherein said multivibrator circuit is abistable multivibrator curcuit.
 12. The apparatus according to claim 10wherein said multivibrator circuit is an astable multivibrator circuit.13. In a liquid crystal display of the type including a commonelectrode, a plurality of electrode segments and a layer of a liquidcrystal material interposed between said common electrode and saidelectrode segments, said layer being capable of changing its response tolight upon activation by a current, the improvement therewith ofapparatus for activating said liquid crystal with a bidirectionalcurrent, comprising: a. a multivibrator circuit having a first andsecond output terminal of the type providing a first given rectangularwaveshape at said first output terminal of an amplitude varying betweena first DC level and a second DC level which second level is relativelyat reference potential, and at second output terminal providing ananalagous waveform of an amplitude at said second DC level when saidfirst waveshape is at said first level and at a said first DC level whensaid first waveshape is at said second DC level, and b. means couplingsaid first output terminal to said common electrode of said liquidcrystal and said second output terminal to at least one of saidelectrode segments to cause a bidirectional current to flow through saidliquid crystal according to said first and second waveshapes.
 14. In adisplay of the type utilizing a liquid crystal having a commonelectrode, a plurality of segment electrodes and a liquid crystalmaterial located between said common electrode and said segmentelectrodes, the improvement in combination therewith of apparatus forproviding a bidirectional current flow through said liquid crystal toactivate the same, comprising: a. a multivibrator circuit having a firstand second output terminal for providing a first waveform at said firstterminal of an amplitude alternately varying between a first DC leveland a second DC level, which second level is relatively at referencepotential, and a second waveform at said second terminal whichalternately varies opposite to said first waveform whereby when saidfirst waveform is at said first DC level, said second waveform is atsaid second DC level, b. means coupling said first output terminal ofsaid multivibrator to said common electrode of said liquid crystal, c.means including a bidirectional current switch of the type exhibiting alow impedance in a first state upon activation of said switch by acontrol signal, and a high impedance in a second state, said switchcoupled in series between at least one of said segment electrodes and apoint of reference potential, and d. control means coupled to saidswitch for providing said control signal to cause said switch to exhibitsaid low impedance state to provide a bidirectional current return forsaid segment electrode.
 15. Apparatus for driving a display of the typeincluding a liquid crystal having at least two electrodes and a liquidcrystal material between said electrodes, said apparatus comprising: a.first and second active devices, each having an input, output and commonelectrode, b. means coupling said common electrode to a point ofreference potential, c. first feedback means coupling said outputelectrode of said first device to said control electrode of said seconddevice, and second feedback means coupling said output electrode of saidsecond device to said control electrode of said first device to causesaid active devices to form a composite multivibrator circuit whereinalternately when one of said first and second devices is biased on, saidother of said first or second devices is biased off, d. means couplingsaid output electrode of said first device to one of said electrodes ofsaid liquid crystal and said output electrode of said second device tosaid other electrode of said liquid crystal to cause a signal to bedeveloped across said liquid crystal according to the states of saidactive devices, and e. means coupled to at least one of said controlelectrodes of said active devices to determine the rate at which saiddevices are biased on and off.